The invention relates to a caterpillar-mould casting method as claimed in the characterizing portion of claim 1, to a casting machine as claimed in the characterizing portion of claim 4, and to a method of changing the blocks of a casting machine as claimed in claim 26.
Machines of this type are used in the continuous fabrication of billets and bands, hereinafter referred to as ‘strand’, consisting in particular of aluminium and its alloys, but also of other materials such as zinc, copper, brass, and steel, as well as a number of non-metallic materials.
The development of methods and apparatuses for this purpose goes back to the previous century and even the century before. Reference is made here to the works of E. Hermann, “Handbuch des Stranggiessens” (Handbook on Continuous Casting), 1958, and “Handbook on Continuous Casting”, 1980 (Aluminium Verlag, Düsseldorf). As these works show, there have been conceived, among other types, casting machines in which the casting mould where the solidification of the melt takes place is formed by metal blocks extending over the width of the mould.
In order to minimise friction between the solidifying melt and the casting mould, the blocks move along with the solidifying strand at the same speed until they reach the end of the mould where they are detached from the strand and directed, by means of chain wheels or arcuate tracks, to the rear of the machine body where they undergo a second change of direction so as to be redirected again to the entry of the mould.
Said blocks may be made of antimagnetic or ferromagnetic material, preferably copper or aluminium, or of cast iron or steel, depending on the particular operating requirements.
In American terminology, casting apparatuses of this type are referred to as machines with caterpillar-mould or as block-casters.
Mounted on caterpillar tracks and moved by a transport mechanism, these blocks circulate around a machine body, with one design including two opposed machine bodies which are positioned in such a way that the distance between the walls facing one another in the mould corresponds to the thickness of the strand to be cast, taking into consideration the shrinking of the melt as it solidifies.
Another design is distinguished by the fact that the machine includes only one machine body around which a caterpillar circulates, the melt being poured onto the caterpillar where it continuously solidifies into a strand. Preferably, the solidifying strand is covered by a gas schrouding in order to prevent unwanted oxidation from taking place on the free upper surface of the solidifying melt.
The following description refers in particular to machines having two opposed machine bodies and two caterpillars. As far as the design and function of the machine bodies and the caterpillars are concerned, the novelty described hereinafter is also applicable to machines having only one machine body provided with a caterpillar moving around it.
In operation, the melt prepared in a furnace flows through a channel into a trough arranged on the inlet side of the mould which extends over the width of the mould and in which the metal level is kept at a required height through controlled material supply. From here, the melt is led through pouring nozzles into the mould which is delimited on the entry side by said nozzle, on the exit side by the solidifying strand and laterally by side dams. The casting direction may be vertical, horizontal or inclined.
The speed of the strand leaving the mould depends on the material and the thickness of said strand as well as on the physical properties of the block material and on the temperature thereof at the entry of the mould. The strand thickness usually obtained with caterpillar-mould casting machines is between 1.5 and 3 cm, preferably 2 cm. The speed of the strand when leaving the machine must be controlled and adapted depending on the particular operating conditions and is normally between 2 and 12 m/min. After it has left the machine, the produced strand undergoes further manufacturing processes in a manner known in the art.
In the section where they form the mould, the blocks are in contact with the melt, taking up the heat to be absorbed therefrom, and are then cooled by means of an aqueous coolant while travelling around the machine body. Experience has shown that the thickness of the blocks is between three and five times the thickness of the strand to be cast, depending on the amount of heat to be stored.
For reasons of a purely physical nature, known caterpillar-mould casting machines are flawed by one big problem.
The unilateral heating to which they are exposed when passing through the mould section causes the blocks to be deformed in an unwanted manner, i.e. they will become subject to distortions the degree of which is aggravated with an increasing length of the blocks. This will cause the walls of the mould to become uneven, which in designs known so far leads to the formation of local gaps between the mould wall and the solidifying strand. In addition to causing an uneven thickness of the strand produced, these gaps will lead to an uncontrollable heat flow from the melt into the mould wall, giving rise to excessive local thermal stresses in the solidifying material which may in turn lead to intolerable cracks in the nascent structure of the strand. In addition, the joints at the abutting faces of the successive blocks tend to become loose, which causes ledges and fins on the surface of the strand, as the melt flows into the interstices and gaps formed in the mould wall.
There is also the problem of sealing the casting nozzle protruding into the mould, as a reverse flow of the melt must imperatively be prevented. The sealing will evidently be the more difficult to achieve the more the blocks are deformed.
Heat stresses increase considerably if the subsequent cooling of the blocks takes place on the surface that has previously been in contact with the melt, hereinafter referred to as front surface.
Depending on the height of the temperature difference between heated and cooled surfaces, the compression stresses and tensile stresses periodically occurring on this surface may fall out of the elastic limit of the block material, which due to material fatigue leads to reticulated cracks on the front surface of the blocks, which in turn has a negative effect on the surface of the cast product—a circumstance which requires an exchange and a remachining of the blocks used after a relatively short operating period.
Due to the above-mentioned high thermal stress experienced by the blocks, the latter are generally to be considered as wearing parts that have to be periodically replaced by remachined or new blocks.
Although caterpillar-mould casting machines are provided with evident advantages over other continuous casting processes as far as the quality of the product is concerned, machines of the types used so far—due to the problems mentioned above, and with the exception of the machine described hereinafter-have been able to stand their ground only for the production of relatively narrow strands, since the problems described dramatically increase with the use of moulds having greater widths.
From the U.S.-Patents U.S. Pat. No. 3,570,586 and U.S. Pat. No. 5,979,539, apparatuses are known which try to prevent warping of the blocks even when used with wide-moulded machines by mounting the beam-like blocks reaching over the width of the mould by means of strong fasteners onto rigid steel supports having practically a constant temperature and the geometrical moment of inertia of which is a multiple of that of the blocks, whereby an excessive deformation of the blocks can largely be avoided. The cooling of the blocks is done during their return travel by spraying an aqueous coolant onto the mould walls. This known concept allows a considerable increase in the machine width as compared to other designs, making it possible, with the use of new or remachined blocks, to manufacture for a limited period of time alloyed aluminium strands of good quality with a width of up to 1.8 m. This result is due to the fact that not only the evenness of the mould walls is maintained by force, but their temperature can be well controlled, due to the relatively high mass of the blocks, by adjusting the cooling system, which allows the solidification process of the material to be cast to take place in an optimal way, so that besides the achieved product quality improvement it is also possible to work with a wider variety of metallic materials and their alloys.
Years of experience have shown, however, that the aforementioned problems have only been partially solved with the concept of the support-mounted blocks just described. Since the blocks are prevented from being deformed upon temperature changes, heavy internal strains will build up in them, according to the laws of physics concerning the strength of materials, and these forces will interfere with thermal stresses of the same modulus which are present anyway in the mould walls so that material fatigue accompanied by the formation of cracks is dramatically accelerated. As subsequent to the heating of the blocks, the same surface that used to be in contact with the melt is sprayed with coolant, the aforementioned effect is even considerably aggravated. In addition, experience shows that the blocks, despite their fixation on rigid supports will nevertheless, after a certain operating time, show distortions, which have a negative influence on the quality of the product, as discussed above. The inadmissibilities mentioned above will result in the need for the blocks to be changed after a relatively short period of operation which, due to the strong fixation of the blocks on the heavy supports, is a very labour-intensive task which involves substantial machine downtime and thus represents a substantial drawback as far as the economy of the plant is concerned.
From the findings available and from experience with caterpillar-mould casting machines, it becomes evident that a further development of these machines in order to solve the problems still remaining is of great importance to the industrial branch concerned, since the casting method in question—once properly improved—provides manifest advantages over other casting types as far as economic aspects as well as the diversity of materials and alloys to be treated and the quality of the product are concerned. It was found that successful operation is only possible if the following conditions are fulfilled:                A) The design concept of the machine must be appropriate for the production of high-quality strands of any width required by the industry.        B) The changing of the blocks must be possible within a small fraction of the time it currently takes, in order to limit the amount of work involved and the downtime of the entire production plant to a minimum.        C) The service life of the blocks must be substantially increased as compared to their current useful life.        
Experience shows that in order to meet the requirement of a uniform solidification process of the nascent strand and, consequently, of a proper heat flow from the latter into the wall of the mould, the deformation of the blocks during their passage through the mould must not exceed one or two tenths of a millimeter, depending on the solidification characteristics of the melt.
According to the laws of physics, the change in absolute shape and dimensions of a free body depends on the size of said body, the expansion coefficient of the material in question, and on the temperature conditions involved. If, for example, an elongated body with a rectangular transverse section, which is the form of the blocks of conventional caterpillar-mould casting machines, is characterised with respect to the central line extending in the longitudinal direction of the body by an asymmetrical temperature profile over its cross-section, said body will react with a deflection. While the length measure of a long body as compared to a short body will increase in a linear manner, the increase in terms of the absolute measure of the deflection is approximately raised to the second power of the aspect ratio of the compared bodies, with the cross-sections of the respective bodies and the temperature profile remaining identical.
U.S. Pat. No. 3,570,586 discloses a practice that consists in subdividing the beam-like blocks extending over the width of the mould into relatively small segments, referred to hereinafter as block elements, to join them together in the lateral direction by means of connecting rods, and to mount the blocks formed in this way, the rigidness of which is reduced as compared to one-piece blocks, onto rigid support members of practically constant temperature, which makes it possible to largely avoid deformations due to temperature changes of the blocks.
This known block structure, however, turns out to be too costly since the blocks are wearing parts that are to be replaced at regular intervals. In addition, it appeared that in operation the elements are displaced due to the constant changes in temperature, so that in the long run the required evenness of the blocks cannot be ensured. The expected success of this supposed solution is thus to be considered as illusory and this method is not apt to go into operation.
In addition, as mentioned above, the concept of exchanging the blocks involves a great amount of work and, consequently, an extended downtime period of the entire production line, particularly due to the necessity to detach said blocks from their support members and to re-mount them thereon. In addition, due to their strong fixation upon their respective support member, the block elements are prevented from undergoing free deformation upon temperature changes, which, as mentioned before, causes additional stresses within the elements and thus negatively influences their service live.
The invention is intended to provide a remedy for this. It is accordingly an object of the invention to provide a casting machine that is capable of fulfilling the conditions mentioned under (A), (B), and (C) for an economically successful utilisation of caterpillar-mould casting machines.